We used to have a local SMTP relay set up that sent all email through microsoft SMTP. We configured Nintex to use this local relay. There were some changes to the authentication on the Microsoft side so email notifications stopped.
Does anyone have Nintex Workflow for SharePoint (2019) configured to use Microsoft365 as the SMTP server? If so, what settings are you using. The account that we use to send email has an M365 account.